1. Field of the Invention
The present invention relates to a method of ultrasonically cleaning a workpiece, and more particularly to a method of ultrasonically cleaning a workpiece by supplying a deaerated cleaning solution to a cleaning tank with ultrasonic vibrator mounted on its bottom, and radiating ultrasonic energy from the ultrasonic vibrator into the cleaning solution to clean a molded, cast or machined workpiece immersed in the cleaning solution.
2. Description of the Prior Art
Workpieces such as ground, bored, or abraded metallic workpieces, ground glass or ceramic workpieces, or injection- or extrusion-molded plastic workpieces are often burred immediately after they are formed. Surfaces of such workpieces are smeared by solid foreign matter such as chips, small broken pieces resulting from burrs, and dust particles. To finish these workpieces, it is necessary to remove the burrs and solid foreign matter off their surfaces and clean the surfaces.
Heretofore, it has been customary to clean machined workpieces with a cleaning solution such as an organic solvent of carbon chloride, e.g., perchloroethylene, 1,1,1-trichloroethylene, or the like, or an organic solvent containing chlorofluorocarbon. Though another separate process is necessary to remove burrs which have not completely separated from the workpiece, the above cleaning process is highly effective to remove foreign matter deposited on the workpiece because the foreign matter can be cleaned off simply by immersing the workpiece in a cleaning tank filled with the organic solvent.
However, organic solvents of carbon chloride are difficult to handle because most of them have an anesthetic effect and tend to cause blood related problems if inhaled over a long period of time. It is also pointed out that chlorine contained in molecules of organic solvents containing chlorofluorocarbons are responsible for destroying the ozone layer around the earth. An international agreement has been reached to abolish the use of all organic solvents containing chlorofluorocarbons.
In view of such drawbacks of the conventional cleaning solutions, research efforts have been directed to the use of an aqueous cleaning solution. It is known that the cleaning solution used in an ultrasonic cleaning process has an increased cleaning effect if it is deaerated to reduce the content of dissolved gas therein. The principles behind the increased cleaning effect of such a deaerated cleaning solution are as follows:
In the ultrasonic cleaning process, partial vacuums are formed in the cleaning solution due to cavitation when ultrasonic energy is radiated into the cleaning solution. Since the cavities formed in the cleaning solution contain only a slight amount of vapor of the cleaning solution and are mostly vacuum, they are immediately collapsed under the pressure of the surrounding cleaning solution. When the cavities are collapsed, microjets are developed in the cleaning solution. Inasmuch as the microjets act on the surface of a workpiece to be cleaned which is immersed in the cleaning solution, solid foreign matter deposited on the workpiece is removed, thus cleaning the workpiece.
If the cleaning solution is not deaerated and contains a high concentration of dissolved gas therein, then the gas is evaporated in the cavities, resulting in the creation of gas bubbles in the cleaning solution. If such gas bubbles are generated, then since the pressure of the gas in the gas bubbles acts against the pressure of the surrounding cleaning solution, the cavities are less liable to collapse, resulting in difficulty in producing microjets. Even if microjets are produced, they are dampened by the gas bubbles, and act less effectively on the surface of the workpiece. Once the gas bubbles are produced, ultrasonic energy radiated by the ultrasonic vibrator is absorbed by the gas bubbles, making it difficult to cause cavitation. Consequently, the ultrasonic cleaning process which employs a cleaning solution that is not deaerated is unable to produce any cleaning effect other than a very weak cleaning effect provided by the gas bubbles.
On the other hand, if a deaerated cleaning solution is employed in an ultrasonic cleaning process, stronger microjets are developed because a smaller amount of gas is evaporated in the cavities and exerts a lower surrounding pressure against the pressure of the cleaning solution.
The inventor has found, as a result of research activities with respect to the ultrasonic cleaning process based on the above knowledge, that since powerful microjets acting on the surface of a workpiece to be cleaned are produced upon collapse of cavities in a deaerated cleaning solution, an ultrasonic cleaning process employing an aqueous cleaning solution is effective in removing solid foreign matter off the surface of the workpiece, and much stronger microjets generated in the aqueous cleaning solution are capable of removing burrs that have not fully been separated from the workpiece.
Even when the amount of dissolved gas in the aqueous cleaning solution is greatly reduced, however, the aqueous cleaning solution may fail to provide a sufficient cleaning effect depending on the type of workpiece to be cleaned.